1. Field of Invention
The present invention relates in general to suppressing electric and electromagnetic interference and more specifically to preventing interference from affecting electronic circuitry that controls an operation such as the air mixture of gasoline in a catalytic converter of an automobile.
2. Description of the Prior Art
Due to stringent limitations on air pollution, automobile exhaust is strictly controlled in order to minimize pollutant levels. A catalytic converter is used to cleanse exhaust gases. The efficiency of the catalytic converter is heavily dependent upon a strictly controlled air/gasoline mixture. Several parameters are considered and manipulated by a computer which then acts to adjust the air/gas ratio and minimize pollutant levels. Typically, electromagnetic sensors, piezo-resistant sensors, Hall effect sensors, CTN type probes, and oxygen probes are used to collect data. The data is sent to a calculating device, such as a computer, where the data is processed and used to drive the ignition and fuel injection of the engine.
Because an electronic computer is used to manipulate the data collected by the various sensors, it is subject to electric and electromagnetic interference that may affect the results generated by the computer, adversely affecting the efficiency of the catalytic converter. The very probes used to collect the data necessary for the computer to operate create interference signals that upset the accuracy of the calculating device. Interferences from the environment external to the vehicle also upset the accuracy of the calculating device. The external interferences can also interfere with the sensors and probes as data is collected, adversely affecting the individual component as it attempts to collect data.
The interference signals disturb the engine's performance, resulting in inefficient fuel consumption and pollution. Currently there is no solution to the problem other than replacing components, which results in high operating costs for the user and high warranty costs for the manufacturer.
Electrical signals are subject to contamination from interference generated by many different sources. The signal can be filtered, or treated, in an attempt to minimize the "noise" generated by interfering signals and isolate the "useful" signal. Interference that is within the same frequency band as the "useful" signal is more difficult to treat. U.S. Pat. No. 4,081,740 to Teratani et al. discloses a capacitor connected between a voltage divider line and ground, distinct from the chassis ground. The capacitor absorbs high frequency noises appearing over the voltage divider line and ground, yet does not address amplitude differences that occur in noise interference. Likewise, U.S. Pat. No. 3,256,487 to Sinninger and French Patent No. 93-06391, which was filed in the United States, disclose a device that acts to maximize the noise to signal ratio of a system by injecting a phase opposition signal against the interference signal, which would be disastrous if used in the asymmetric system of an automotive vehicle. Injecting a phase opposition signal would double the effect of the interference voltage rather than cancel out its effects as in the present invention. The interference suppression system as taught by Sinninger requires that the phase relationship of the interference signals be different from each other, ideally 180.degree. out of phase. Likewise, in French Patent No. 93-06391 discloses a filtering device including two light emitting diodes in parallel, and a processing device that injects a phase-opposition current on the ground parts of the machine to eliminate higher-amplitude interference. The teachings of French Patent No. 93-06391 are not only opposite the teachings of the present invention, but are functionally impossible. The processing device of the French patent is a capacitor. A capacitor cannot process amplitude, and the diodes disclosed do not filter. And as discussed above, injecting a current in phase opposition effectively doubles the interference signal rather than cancels it out.
In automotive systems, there are many components that come together and operate as a complete assembly. Individually, each of these components must meet an electromagnetic compatibility (EMC) standard. However, when the individual components are combined, the assembly may not meet the EMC standard. In addition, automotive manufacturers rely on the metallic parts of the car body as a ground, when in fact they are not. The "ground" of the car body is subject to interference generated by galvanic coupling, or conduction of current through one or several impedances that are common to two or more pieces of electrical equipment. All conductors have an inherent impedance, which is mostly negligible at low frequencies. However, harmonic currents will develop harmonic voltages through an impedance that increases as frequency increases. The harmonic voltages interfere with the "useful" signal, adversely affecting the efficiency of the catalytic converter. Magnetic and capacitive coupling can also interfere with the "useful" signal.
What is needed is a simple and cost effective device which adequately neutralizes the effect of an interference signal on an asymmetric system.